Chagas disease istheresultofinfectionbytheparasiteTrypanosoma cruzi. About18millionpeopleareinfectedworldwide,andapproximately50,000dieeachyearasaresultofcomplicationsfromthedisease.Themostseriousformsinvolveenlargementoftheesophagus,colon,andheart.Lossofparasympatheticcontrolisasignificantcomponentoftheinitialstagesofthedisease;shortlyaftertheinitialinfection,theparasympatheticneuronsinnervatingtheheart,esophagus,andcolonaredestroyed,whichleadstoarrhythmias(andpotentiallysuddendeath)andaperistalsis.Chroniccardiomyopathy(malfunctionoftheheartmuscle)thatcanleadtodeathoccursinapproximately30%ofthoseinfected.Althoughthepathogenesisofthecardiomyopathyisnotfullyunderstood,oneleadingideainvolvesautoimmunity.Antibodiesagainsttheparasiticantigenshavebeenfoundtobindtotheβ-adrenergicandM2acetylcholinereceptorsintheheart.Theseantibodiesnotonlytriggerautoimmuneresponsesthatdestroyheartmusclebutalsoactasagonistsatthesereceptorsandcauseinappropriateresponsesofthecardiovascularsystemtochangingexternaldemands.
Central Control of Autonomic Function
The discharges of autonomic preganglionic neurons are controlled by pathways that synapse on autonomic preganglionic neurons. The pathways that influence autonomic activity include spinal cord and brainstem reflex pathways, as well as descending control systems originating at higher levels of the nervous system, such as the hypothalamus.
Examples of Autonomic Control of Particular Organs
The dilator and constrictor muscles of the iris, which are under the control of sympathetic and parasympathetic fibers, respectively, determine the size of the pupil. Activation of sympathetic innervation of the eye, via thoracic white rami and sympathetic trunk ganglia, dilates the pupil, which occurs during emotional excitement and also in response to painful stimulation. The neurotransmitter at the sympathetic postganglionic synapses is norepinephrine, and it acts at α receptors.
The parasympathetic nervous system exerts an action on pupillary size opposite that of the sympathetic nervous system. Whereas the sympathetic system elicits pupillary dilation, the parasympathetic system constricts the pupil. The preganglionic parasympathetic nerves that innervate the pupillary constrictor are in the Edinger-Westphal nucleus, which is in the midbrain, and travel in cranial nerve III, and so damage to this nerve can lead to a dilated pupil (mydriasis).